Dielectric heating means with automatic compensation for capacitance variation



. SCHUMAN 2,467,782

April 19, 1949. A

DIELECTRIC HEATING MEANS WITH AUTOMATIC COMPENSATION FOR CAPACITANCE VARIATION Filed Sept. 20, 1947 J 6? 7a 76 as WITNESSES: TNVENTOR H8 -H/beri Schuman 1 6. BY Y ATTORNEY Patented Apr. 19, 1949 DIELECTRIC HEATING MEANS rrn TOMATIC COMPENSATION FOR oArAcI, TANCE VARIATION Albert Schuman, Glendale, Calif assignor t Westinghouse Electric Corporation, Eastlittsburgh, Pa., a corporation of Pennsylvania Application September 20, 1947, Serial No.'775,'1i6

1 Claim. 1

,Myqinvention is generally related to the dielec- =tric heatetreatment of materials, such as laminatcd gplastics, resins, preforms, or the like, which undergo a change in dimensions during heatvtreatment. More particularly, my invention is directed to improvements in apparatus for the treatment of such materials under heat and pressure, the heat being introduced dielectrically by meanszof spaced relatively insulated heating-electrodes on opposite sides of the material; the heating-electrodes preferably being pressed toward eachother.

Dielectric heating generally requires high-fre- ;guency,power in the order of 5-50 megacycles and morepand in order to get a satisfactory transfer ofpower from the generator or other source of high freguency into the material between the relatively insulating heating-electrodes, it is desirable to have the heating-electrodes connected in orpart of a branch circuit or network which iswtuned to the frequency of the high-frequency power supply or which is impedance matched to asource of high-frequency power supply such as a high-frequency transmission line. However, during heat-treatment, the physical characteris- ,tics of .a material may change, as for example, bya change in the thickness of the material between the heating-electrodes, as the material is being heat-treated. This is especially true of Dreforms or preliminary substances containing blowingagents. Such change affects the effective capacitance provided by the spaced heating-electrodes. The change in the capacitance maybe great enough to seriously disturb the tuning or matching, and hence the amount of power delivered to the material.

.An object of my inventionis to provide a dielectric .heating system of a type described in which changes in the capacitance between its spaced relatively insulating heating-electrodes because .of changes .in the thickness of material vbeingheat-treated, .are automatically compensated for, at least in part, by a combined mechanical. and electrical mechanism which is directly 'responsiveto the changing thickness of the material.

HIn general, ,inaccordance with my invention, the heating-electrodes are part of a tuning or :matching network which is provided with an auxiliaryadjustable reactor, either an inductance coil-or capacitor. This reactor is varied bya member whichmoves. in response to changes in a :dimension of the material being heat-treated. Thewmemberymay ,be one of the heating-electmdes,;-,or.-,a :,distinct;, device. :Movement of the member changes the variable reactor preferably so as to keep theinductance and capacitance relations in the network at some predetermined value or values, although the capacitance between the heating-electrodes changes as the material undergoes a physical change inits size.

My invention is especially useful in connection with a dielectric heating system in which the material is heat-treated between two platens of a press, In such case, themember controlling the variation in the auxiliary reactor can be mechanically connectedto a movable platen of the press, or can be placed on orin the material so that its position is changed when the size of the mat rial h es- An object o m inve t o s to provide a ielectric heating system of a type described, in whicha variable dielectric load, consisting of the heating-electrodes .andall of the material therebetween, automatically controls a reactor for maintaining the'efiiciency of the transfer of highfrequency power tothe load.

Features, innovations, methods and combinations of my inventiomin addition to the foregoing, will be discernible from the following description and generally schematic drawing of several embodiments of the invention.

In the drawing:

Figure 1 is a wiring diagram of an embodiment of my invention which a heating-electrode is mechanically connected to a movable plate of a variablecapacitonthe capacitor being electrically inseries with the dielectricload;

Fig. Zis a wiring diagram of a modified form of my inventionas more particularly related to .a press;

Fig. 3 is aview-takensubstantially along the line III--III of Fig. 2;

Fig. 4 is a wiring'diagram of a further modified formof vmy inventioninwhich the movement of aplatenis amplifiedior controlling a capacitor in parallelwith the. dielectric load;

Figs.c5 and 6 are wiring diagrams of further modified .forms of my invention in which the dielectric loadchanges avariable inductance coil, Fig. 5 showing the coil electrically in parallel with the load, and Fig. dshowing the coil in series with the load; and

Fig-'7 is an abbreviated view of a still further form of my invention in which an air gap is provided inserieswith the material being heattreated.

.My invention can be-conveniently described in connection with plastics ofa type madefrom layers t ofpaper, cloth or. other substanceswhich are first impregnated with a resin and subsequently cured under heat and pressure; the heat bein supplied dielectrically while the substance is in a press. Preforms for plastic molding are a further example of materials which usually undergo such changes. Preliminary to heat-treatment, the impregnated paper etc., is compressed between the platens of the press to as much as of its original size. The heating-electrodes in the press, which may comprise the platens themselves, and the material therebetween form a dielectric load, This dielectric load has the effect of a capacitor and is usually part of a tuned network or part of an impedance network, depending on whether the network is energized directly from a tube-oscillator generator, or through a high-frequency transmission line such as a coaxial cable. Usually the heat-treatment is accompanied by a change in the thickness and :in the loss factor of the material. By loss factor is meant a combination of the power factor :and dielectric constant of the material. Hence, the capacitance represented by the dielectric load :alters materially as the material heats or cures, :so that the relationship of the inductance, L, and capacitance, C, in the network of which the load :is a part, is disturbed.

It is possible to continuously retune or rematch -the L-C relations of a network by manual operation of a reactor connected to the dielectric load, but such a method has limited application because it may require an operator to stand near high-voltage high-frequency equipment. Experience has shown that the greatest change in the capacitance of the dielectric load and the largest change in the thickness of the material occur simultaneously during the first part of the heating period of a material such as resin-impregnated paper. The changes are very slow during the remainder of the heating period. The change in the thickness of the material can refiect itself as a change in the spacing between the press platens. In accordance with the teachings of my invention, the change in thickness controls an auxiliary adjustable reactor in the network of which the dielectric load is a part, for circuit tuning or matching,

Referring more particularly to the embodiment shown in Fig. 1, the dielectric heatingload iD consists of a pair of spaced relatively insulated heating-electrodes 2 and 4, having dielectric ma- .terial or work 6 in the space therebetween.

Usually the heating-electrodes are metallic plates which are relatively movable. To obtain such an arrangement, one of the heating-electrodes may be fixed and the other movable. To illustrate the fixed heating-electrode in the drawing, slanted lines have been associated therewith on the side away from the material being treated.

In Fig. 1, the heating-electrode 2 is fixed and the other heating-electrode 4 is movable and rests on the material 6. Directly connected to the heating-electrode 4 is a movable bar member 8, which in turn is directly connected to a capacitor plate or plates III of a variable capacitor I2. The other capacitor plate or plates I4, which are insulated from the capacitor plate or plates I0, may be stationary.

The movable member 8 may be of metal so as to be a conductor of electricity. Hence, the member 8 essentially is a mechanical and electrical interconnection between a movable part or member of the heating-electrode' l and the movable part I of the capacitor I2. However, a separate loose conductor I6 may be provided 4 between the heating-electrode 4 and the capacitor plate or plates III for the electrical current therebetween.

The dielectric load D and the capacitor I2 are connected in series to a secondary coil I8 which is coupled to an inductance coil 20 of a tank circuit 22 that is part of a tube-oscillator generator 24.

The network comprising the secondary coil I8, the capacitor E2, the dielectric load D, and the connections therebetween, can be adjusted in accordance with the frequency of the tube-oscillator generator 24 so that the desired voltage is present across the heating-electrodes 2 and 4. However, as the work 6 of the dielectric load D heats, its thickness and electrical properties change. A change in the thickness of the work raises the heating-electrode 4 which in turn lifts or moves the capacitor plate or plates III of the capacitor I2. The network can be kept in balance by so designing the capacitor I2 that its capacitance changes in an opposite sense to the change in the capacitance of the dielectric load D; that is, as the capacitance of the load decreases, the capacitance of the capacitor I2 increases.

In Fig. 1, the heating-electrode 2 has been shown grounded. This means that the heatingelectrode 4, the movable member 8, and the capacitor I2 must be insulated from ground. If a connection I6 is used, it should also be insulated.

2 shows an embodiment of my invention applied to a press, in sufiicient detail to one skilled in the art. The press comprises bottom and top platens 30 and 32 respectively. The platen 3B is stationary, and the platen 32 is resiliently pressed toward the platen 30 by means of a pneumatically operated ram which is schematically represented at 34. Two layers of work 35a and 362; are provided in the space between the platens 3i] and 32, the layers being separated by a third platen 38.

Electrically, the top and bottom platens 30 and 32 can be grounded and the intermediate platen 38 insulated so that, in efiect, the surface of each platen, which faces another platen, provides a heating-electrode. The intermediate platen pro-. vides a bottom surface 40 facing the lower platen 30 and an upper surface 42 facing the top platen 32. The facing surfaces terminate the electric fields between the associated facing platens when high-frequency voltage is across the platen 38 on the one hand and the platens 30 and 32 on the other.

High-frequency power is derived from a tube+ oscillator generator 44 having a tank circuit 54 to the inductance coil of which is coupled a coil or winding 56, one end of which is grounded as at 51. The other end of the coil 58 is connected by a conductor 58 to a stationary plate or plates 60 of a capacitor 62 having movable plate or plates 64 which are relatively insulated from the stationary capacitor plates 80. The movable capacitor plate or plates 64 are electrically con nected to the insulated platen 38 through a flexible conductor 68. A rigid bar It! is mechanically connected to the platen 33 and contacts a bar I2 through which the capacitor plate or plates 64 are moved.

When the work 36a and 36b between the platens absorbs energy, it changes in thickness, thereby raising the bar I0 because the ram 34 resiliently presses on the platen 3'2 with a constant pressure as is common in such presses. The bar I0 raises the bar I2, thus causing the capacitor plate or plates 64 to alter their overlap with the stationary capacitor plate or plates 60. The resulting change in capacitance will depend on the configuration of the peripheries of the respective plates. This is symbolically shown in Fig. 3 where the peripheries of the capacitor plates 60 and 64 are irregularly shaped in accordance with any desired mathematical formula so that a predetermined displacement of the platen 38 will cause a predetermined change in the capacitance of the capacitor 62.

Fig. 4 shows a further modified form of my invention in which the auxiliary reactor is electrically in parallel with the dielectric load D, and is operated by a grounded platen, in this case a top platen Id. The press of Fig. 4 is essentially the same as that shown in connection. with Fig. 2; but in Fig. 4 the movable member lo, which corresponding to the bar of Fig. 2, is attached to the top platen I l, operates on a crank arm 18 that turns a shai t 80 in a speed-changing mechanism 82. The speed-changing mechanism 82 has a shaft 84 that turns a series of movable plates 86 with respect to a plurality of stationary plates 88 of a capacitor 90. By means of the speed-changing mechanism 32, a linear movement of the platen I4 is changed to a rotary movement of the capacitor plates 86. The speed-changing mechanism can move in direct proportion to that of the platen I4 or may include a cam arrangement for any desired motion change.

The particular network or networks and the connections to be used in them are known to the art, and do not form a part of my invention except insofar as a separate variable reactor is provided and is varied in accordance with a mechanical displacement of a member responding to a change in size or thickness of the material being heat-treated.

Instead of having a capacitor in series or in parallel with the dielectric heating load, as shown in Figs. 1-4, it is possible to use networks in which a variable inductance coil is arranged to be changed by displacement of a heating-electrode or platen. A variable inductance coil operable by a movable platen is indicated in Figs. 5 and 6; the coil being in parallel with the dielectric load in Fig. 5 and in series therewith in Fig. 6.

In Fig. 5, the grounded heating-electrode or platen 92 is movable while a fixed insulated electrode 93 is electrically connected to a manually variable inductance coil 04 which is electrically connected to an insulated conductor 96 of a highfrequency transmission line 98 having an outer grounded conductor I00. A variable inductance coil I02 is electrically in parallel with the dielectric load D, the electrical connection including a rigid metal bar member I04 movable with the grounded platen 92. The bar member I04 controls the position of a tap member I06 along the turns of the variable inductance coil I02.

In Fig. 6, a rigid insulating movable member I08 is rigidly mechanically connected to an insulated platen H0. The member I08 moves a movable tap II2 of a variable inductance coil H4 upwardly as the work IIB expands. A flexible conductor III electrically connects the tap II 2 to the platen H0. The inductor H4 is electrically connected in series to a variable coil II8, which is electrically connected to the insulated conductor I of a coaxial transmission line I22. A manually variable capacitor I24 is electrically in parallel with dielectric load D for completing the network from the insulated conductor I20.

It is obvious that the mechanical and electrical connections from a movable platen to the auxiliary variable reactor can be simply a simple metal bar, or the electrical and mechanical connections can be separated. Moreover, either the electrical or the mechanical connection, or both, can be accomplished by a part consisting of a plurality of independent members or a single part the portions of which can be considered as distinct members.

It is also obvious that the moving member which rests on the material or is otherwise responsive to the movement of the material, need not necessarily be a platen or a heating-electrode. This is indicated in Fig. 7 where two stationary relatively insulated heating-electrodes I30 and 532 are spaced to receive work I34 in the space therebetween; the work, however, not fully occupying the space so that an air gap I 36 is provided between the top of the work and the upper heating-electrode I32. A movable member I38 rests on the material I34 so that it rises as the material heats. This movement of the movable member can control the reactance of a variable reactor in any suitable manner.

Since numerous changes may be made in the above-described constructions and electrical citcuits, it is obvious that different embodiments of the invention may be made without departing from the spirit and scope thereof. It is intended that all matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense, unless otherwise required by the prior art and claims.

I claim as my invention:

Dielectric heatin means comprising, in combination, a pair of spaced relatively insulated heating-electrodes adapted to receive material therebetween of a type which changed its thickness between said heating-electrodes during heat-treatment, means comprising an electrical network for applying a high-frequency voltage across said heating-electrodes, whereby to provide an alternating electric field for heat-treating said material, one of said heating-electrodes being movable by the material as it changes in thickness, conductor means electrically grounding said movable heating-electrode, a mechanism constructed and arranged automatically to follow the movement of said movable heatingelectrode, said network comprising a capacitor having a plurality of relatively insulated capacitor-plates, including a movable capacitor-plate, conductor means electrically grounding said movable capacitor-plate, said mechanism including an interconnection between said movable heating-electrode and said movabl capacitorplate, which moves said capacitor-plate in accordance with the movement of said movable heating-electrode.

ALBERT SCHUMAN.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Number Country Date 385,265 Great Britain Mar. 13, 1931 556,292 Great Britain Sept. 28, 1943 

